AVR Freaks

Well, you seem to have a debugging variable there, "knt". What value does it get?

 I removed the varable knt, no difference.

I see you have enabled interrupts for the USART but I don't see any ISRs?? Maybe lucky you are getting anything at all?

 Interrupts are enabled in CTRLA which is not changed.

Running program printf example on ATTiny817 Xplained Mini

#define F_CPU 20000000ul
#define PRESCALE 1
#define F_PER (F_CPU / PRESCALE)
#define BAUD_RATE0 38400ul
#define USART0_BAUD_RATE (F_PER * 4 / BAUD_RATE0)
#include <avr/io.h>
#include <avr/interrupt.h>
#include <stdio.h>
#include <stdlib.h>
#include <util/delay.h>
#include <string.h>
static int uart_putchar(char c, FILE *stream);
static FILE mystdout = FDEV_SETUP_STREAM(uart_putchar, NULL, _FDEV_SETUP_WRITE);
int main(void)
{
	 //set peripheral prescale to 1
	CPU_CCP = 0XD8;
	CLKCTRL.MCLKCTRLB = 0;   //1
	while ((CLKCTRL.MCLKSTATUS & 1)){};    //wait for clk change
	stdout = &mystdout;
	//SET PB2 FOR OUTPUT FOR TXD
	PORTB.PIN3CTRL &= ~PORT_PULLUPEN_bm;  //PB3 = RXD no pullup
	VPORTB.OUT |= PIN2_bm;  //PB2 out = 1;
	VPORTB.DIR |= PIN2_bm;   //PB2 out
	VPORTB.OUT &= ~PIN2_bm;  //PB2 out = 0;
	//setup USART0
	USART0.BAUD = USART0_BAUD_RATE;
	USART0.CTRLB = USART_TXEN_bm;	//enable tx;
	sei();
	printf("Hello, world!\r\n");
    while (1) 
    {

    }
}

static int uart_putchar(char c, FILE *stream)
{
	while ((USART0.STATUS & USART_DREIE_bm) == 0);
	USART0.TXDATAL = c;
	return 0;
}